1. Field of the Invention
This invention relates to processes and systems for inserting a video image into a graphics image when generating a video signal.
2. Description of Related Art
Conventional computer systems generate pixel maps to represent graphics images. A pixels map is a two dimensional array of pixel values where each pixel value indicates a color for a corresponding pixel (area) on a monitor or other video display. A video encoder generates an output video signal from the pixel values in a pixel map, and a monitor displays the image represented by the video signal. For the image to have the proper appearance, the rows and columns of the pixel map must be synchronized or matched with portions of the output video signal that are associated with the same rows and columns on the monitor so that each pixel is displayed with the color identified by an associated pixel value.
Video overlay systems can insert into a graphics image a video image such as might be generated by a television tuner, a video camera, VCR, or a video decoder. Video overlay systems commonly include software that generates a pixel map representing the graphics image and provides in the graphics image a video window which is filled with a color (or chroma) key. A separate device such as a video capture card generates the video image. In one type of system, an analog video signal represents the video image; and when converting the pixel map representing the graphics image to an output video signal, an overlay system recognizes the chroma key and inserts the analog video signal in place of a signal representing the chroma key. When the graphics signal no longer represents the chroma key, the output signal switches back to the video signal generated for the graphics image. In this type of overlay system, proper synchronization of the inserted video signal in the graphic video signal can be difficult to achieve. An alternative overlay system generates pixel maps representing frames of the video image to be displayed in the video window. The pixel maps for the video image contain rows and columns of pixels matching the area of the video window. When a video encoder encounters in the pixel map for the graphics image a pixel value matching the chroma key, that pixel value is replaced with a pixel value from the pixel map for the video image.
Both of the above overlay systems encounter difficulties when inserting multiple video images into multiple video windows in the graphics image. For multiple video windows, the overlay system must select from among multiple video signals or pixel maps each time a color key is encountered. In particular, the overlay system must match each video window with the video signal or video pixel map corresponding to the video window. Matching windows and video images can be prohibitively complex.